It is common practice to use a light guide (or so-called "light pipe") behind an LCD to distribute light from one or more light sources, such as light emitting diodes (LED's). The light guide is generally in the form of a plate-like member. Particularly in the case of a low-profile assembly, the LEDs are provided, for example, in a recess, at the side faces of the light guide. Light enters the light guide through the side faces and is distributed through the light guide by internal reflection. The front face of the light guide may be "roughened" to make it lossy so that light escapes into the LCD. The light guide thus acts to distribute light over a wide area of the LCD.
However, when relatively few light sources are used, there is a tendency for the illumination to be uneven over the whole area of the LCD. Generally the light is more intense nearer to the LED's and less intense further away.
U.S. Pat. No. 4,975,808 discloses a liquid crystal display apparatus which aims to provide a more uniform distribution of light across a low-profile LCD. In this case the back face of the light pipe is selectively painted white. A pattern in the form of a border, half-sun and ovals around each lamp is left unpainted. The smooth unpainted surfaces of the light pipe internally reflect light that has an angle of incidence less than the critical angle. The white paint provides diffuse reflection, scattering the light. Some of this light escapes the light pipe and enters the LCD. The unpainted portion has the effect of dimming the so-called hot spots caused by the proximity of the light-sources, and increasing the brightness in the painted area. While this arrangement goes some way to improving uniformity of illumination, it still leaves room for improvement. Moreover, the use of painted and unpainted reflected areas provides only limited scope for varying the relative intensity in the dimmer areas insofar as the intensity is enhanced equally in all painted areas and there is little or no control over the degree of intensity enhancement in the painted areas.